This invention relates to a reactive dielectric filler composition. More specifically, the invention relates to a two-part curable, dielectric composition having low initial viscosity, the composition being curable at ambient temperatures to thereby produce a thermally and hydrolytically stable, conformable, soft, moisture-insensitive gel.
The composition of the invention herein has particular utility in the area of cable splices and terminations, junction box fillings, and potting or encapsulation of electronic components. A particular utility for the composition involves the filling and protecting, in conjunction with a suitable closure, of unfilled or pertrolatum-polyethylene filled, reenterable of nonreenterable communication cable splices.
For obvious reasons, it is imperative that a cable splice or termination be maintained in as moisture-free an atmosphere as possible. Furthermore, in conjunction with buried cable splices, such splices are typically contacted by moisture, bacteria, and fungi found in the soil, whereby a filler or encapsulant utilized in conjunction with the splice must be resistant to the growth of such bacteria and fungi, as well as being moisture insensitive.
Presently, one common method for preparing reenterable splices is a hot-pour process, in which an amorphous or semi-crystalline polyolefin, bitumen, or paraffinic wax is heated to a pourable state, typically requiring high temperatures, and then poured into the splice closure. The disadvantages of such a process are obvious, namely the potential safety hazard, the difficulty of proper temperature control, and the possibility of subsequent cable insulation damage. Splice reentry is difficult and requires special tools, unless the splice closure is reheated in order to soften the encapsulant contained therein.
Another process for encapsulating a cable splice involves a composition such as a grease or low molecular weight polyolefin, e.g. polyethylene or polybutylene, which can be pumped into the splice closure with the aid of a caulking gun. In order to eliminate voids in the closure, skilled carftsmanship is necessary to insure that the conductor core bundle is thoroughly packed with the grease prior to the final pumping operation. Also, temperature fluctuations cause such compositions to flow and thereby form channels, which can serve as points of water ingress into the closure. Splice preparation and reentry are time consuming, temperature dependent, and extremely messy.
Another method utilizes a very fine, specially treated, hydrophobic powder, e.g., calcium carbonate, as a reenterable splice encapsulant. In this instance, splice preparation requires no mixing or special tools and can be undertaken independent of ambient temperature. However, in order to eliminate voids within the splice, skilled craftsmanship is required to insure a thorough compaction of the powder into the conductor core bundle. During splice preparation, there is a tendency for the powder to bridge small interstices within the core bundle, which can ultimately lead to void formation due to further powder compaction. In this instance, continuous positive pressure must be maintained on the closure to assure a water-tight seal. Again, the operations of splice preparation and reentry are extremely messy and may be somewhat hazardous from a health standpoint due to the small particle size of the powder utilized.
Still another method utilizes low density rigid polyurethane foams as a reenterable splice filler. In such an application, the conductor core bundle and conductors therein must be protected by a suitable covering from the foam, in order to provide reentry capability. This large central void increases the possibility of water ingress to the splice. Furthermore, foam density is temperature sensitive and it is thereby difficult to accurately determine the quantity of material necessary to fill the splice closure. In order to facilitate splice reentery, it is common to insert a rip-cord into the foam during splice preparation.
At present, one of the most common and presently acceptable methods of preparing reenterable communication cable splices is based on two-part, reactive, typically plasticized, low viscosity polyurethanes, which after mixing on site can be poured or injected into the splice closure, whereupon they cure to a semi-rigid gel possessing relatively low internal strength. These polyurethane systems, however, typically display less than optimum electrical properties. For example, initial insulation resistances of most of such systems have been determined to be in a relatively low range, typically from 10.sup.8 to 10.sup.10 ohms. Desirable insulation resistances for such materials should optimally be on the order of 10.sup.12 to 10.sup.14 ohms.
In addition, component parts within a splice to effectuate the conductor connections are typically manufactured from moled polycarbonate. While polycarbonate molded connectors provide excellent electrical characteristics, one disadvantage thereof is their susceptibility to cracking under the influence of tensile stress and/or strain in an environment, whether the environment be natural or artificial. Molded pieces of polycarbonate typically inherently contain areas of high stress which are known to absorb stress-cracking agents more rapidly than other simpler molded pieces. Stress cracking therein typically results in both swelling of the polycarbonate and crystallization of the amorphous polymer. Such stress cracking may decrease the electrical characteristics of the molded polycarbonate connector to the point where it is ineffective in the splice, thereby drastically reducing service life. To our knowledge, there is presently not available a communication cable splice reenterable encapsulant material which is substantially completely inert toward molded polycarbonate connectors.
It has now been found that all of the foregoing problems can be overcome by the use of a hereinafter defined two-part system capable of providing a crosslinked polyurethane in the form of a soft gel which is substantially compatible with polycarbonate connectors.